Anyone who spends any time reading through the maze of photography related forums and web sites will invariably come across many "facts" regarding cameras, lenses and techniques that are simply wrong yet they are repeated and quoted in almost parrot like fashion until people start to believe them.
What is your favourite "fact" that you see repeated on the 'net or anywhere for that matter that is wrong?
My favourite is that only aperture openings determine exposure levels when using flash / strobe equipment where there is no intruding ambient light.